systemctl: support remote and privileged systemctl access via SSH and pkexec

systemctl: support remote and privileged systemctl access via SSH and pkexec

This adds support for executing systemctl operations remotely or as
privileged user while still running systemctl itself unprivileged and
locally.

This currently requires a D-Bus patch to work properly.

https://bugs.freedesktop.org/show_bug.cgi?id=35230